Heat exchanger for vehicles and process for producing the same

ABSTRACT

A heat exchanger element has a row of multiple channel flat pipes ( 3 ) that are mutually spaced apart and have ends connected in a fluid-tight manner to a tubular connector container ( 6 ) and to a tubular distributor ( 1 ). The ends of the multiple channel flat pipes ( 3 ) are separated into multiple zones ( 3′, 3″ ) shaped in such a way that each zone ( 3′, 3″ ) is parallel at its end on the side of the connection to the main plane of the multiple channel flat pipes ( 3 ), and the zones ( 3′, 3″ ) overlap at least partially in the direction of the longitudinal axis of the collector container ( 6 ) and distributor ( 1 ).

The invention relates to a heat exchanger for vehicles that comprises at least one heat exchanger element having a row of multiple channel flat pipes that are mutually spaced apart and have ends each connected in a fluid-tight manner to a collector container and a distributor. Such heat exchangers, for example, are included in vehicle air conditioning units as condenser/gas cooler or evaporator.

The generic structure of the heat exchanger elements has established itself in today's air conditioning units, aiming at meeting the general requirements of vehicle engineering. Such requirements, for example, are little space, a high degree of operational reliability and functional efficiency, environmental compatibility to an ever increasing extent as well as little manufacturing effort and costs.

The multiple channel flat pipes contain flowing pressurized fluids that dissipate or take up heat and are provided with a multitude of flow channels that are typically arranged parallel, in a row, mutually spaced apart at small distances. This structure is to enable contact of the fluid to the flat pipe body over an area as large as possible, the flat pipe body being passed around by an external medium involved in the heat exchange. The characteristic flat pipe shape, resulting from the arrangement of the flow channels in rows, in heat exchanger assemblies also functions to guide the flow.

In order to enable modern heat exchangers to work using an environmentally friendly, effective refrigerant, especially on the base of highly pressurized carbon dioxide, design modifications are necessary compared to the state of the art to fulfil the growing static requirements and the design demands on the burst pressure that result. Especially parts with large fluid-passed cross-sections and the connections between individual assemblies exposed to fluid pressure as well, have to be built significantly more solid and heavier.

The requirements mentioned manifest themselves, as far as the heat exchanger element is concerned, above all in increased wall thickness of the collector containers, or in the geometric optimization thereof. Therefore, but also for reasons of strength, round pipes have already gained acceptance as collector containers connected to the multiple channel flat pipes. But as increased burst pressure is called for, also this geometry requires increased wall thickness so that for a constant inner diameter, the outer diameter and possibly the construction depth of the entire assembly have to be dimensioned distinctly larger. Moreover, the relatively thick walls and large dimensions of the container and hence of the heat exchanger not only result in big space required, but also in increased weight and higher material demands.

Furthermore, great differences of the material masses, for example, of parts connectable by brazing or welding to be connected to the multiple channel flat pipes, such as the collector container in hand, imply that choosing the brazing or welding parameters required to achieve pressure-resistant and fluid-tight connections may become critical.

Therefore, if high fluid pressures are used, it would be advisable to reduce the large fluid-passed cross-sections. The walls of the collector containers or distributors, particularly in heat exchangers where CO₂ is used as refrigerant, such as in gas coolers for a CO₂-system, must withstand not only to distinctly increased pressures, but also increased temperatures compared to traditional systems.

In conventional heat exchanger elements, the multiple channel flat pipes are connected to collector containers or distributors with circular cross-section after the multiple channel flat pipes have been pushed through slits into the collector container or distributor. For fluid-dynamic reasons, the multiple channel flat pipes will always be aligned such that the longitudinal axis of the collector container or distributor essentially runs through the main plane of extension of each multiple channel flat pipe. This determines the smallest possible inner diameter of the collector container or distributor, because the multiple channel flat pipes must be completely inserted in the respective slits to make the connection tight without individual channels of the multiple channel flat pipes being closed.

Another problem of heat exchangers produced to the state of the art where the inner diameter of the collector container or distributor is only little larger than the width of the multiple channel flat pipes is that fixing-in the multiple channel flat pipes requires the multiple channel flat pipes inserted deep in the collector containers or distributors, if connection is made crosswise to the cylinder axis of the collector containers or distributors. This may lead to fluid flow problems, particularly to inacceptable refrigerant pressure drop in the distributor, and to stability problems as well.

Known are designs of heat exchangers where the ends of the multiple channel flat pipes are twisted partly by up to 90° to the main plane of extension of the multiple channel flat pipes and inserted in corresponding slits that run parallel to the main axis of the collector container or distributor (WO 2005/071340 A1). Thereby the inner diameter of the collector container or distributor can be designed almost independent of the width of the multiple channel flat pipes. However, the minimum distance between neighboring multiple channel flat pipes clearly increases, because the width of the multiple channel flat pipes in the area of connection to the collector container or distributor conflicts with an arrangement as tightly packed as desired. Therefore, effectiveness of the heat exchanger relative to a given mounting space reduces. Further, as the twisted ends of the multiple channel flat pipes act as guide vanes the flow conditions in the connection area change so that heat exchange may be impeded. For smaller angles of twist, corresponding problems occur.

Approaches are known that solve the above problem by dividing the ends of the multiple channel flat pipes and subsequent twisting the partial zones created by the dividing, connecting the twisted partial zones to the collector container or distributor parallel to each other and to the main axis of the collector container or distributor (DE 101 46 824 A1, DE 10 2004 002 252 A1). Thereby the dimensions of the vortexing space and the minimum distance between two neighboring multiple channel flat pipes can be reduced. But these approaches do not generally solve the problem.

In addition, heavy deformations of the multiple channel flat pipes, as caused by twisting the ends by 90°, always involve, if executed in a relatively limited end zone, the danger of damage so that cracks, leaks or constrictions occur.

Further, closely adjoining longitudinal slits in the outer wall of the collector container or distributor should be avoided for reasons of strength, because the static changes of the component implied may require even thicker material in the wall region.

Today, one of the prevailing problems when heat exchangers, particularly gas coolers in vehicles are designed is finding suitable forms of connection between the inserted multiple channel flat pipes and the respective collector containers or distributors, especially with acceptable dimensions and values of the refrigerant's pressure drop in the distributor.

The invention aims at proposing a heat exchanger for vehicles that while having outer dimensions as small as possible is established suitable for use in air conditioning units of vehicles at high operational pressures, particularly in CO₂-systems. The disadvantages of prior art should be avoided, and a suitable process of producing the heat exchanger according to the invention should be described.

The problem is solved by a heat exchanger having the features of claim 1. Further developments and advantageous embodiments of the invention are described in the sub claims. Claim 8 discloses a process for producing the heat exchanger according to the invention, while the process is further established in the dependent claims 9 to 14.

The invention is based on a heat exchanger element with a row of multiple channel flat pipes mutually spaced apart, with the ends of the multiple channel flat pipes being connected in a fluid-tight manner to a tubular collector container and a tubular distributor and divided into several zones shaped such that the zones with their ends directed toward the connection each are aligned parallel to the main plane of extension of the multiple channel flat pipes, while the zones at least partly overlap in direction of the longitudinal axes of the collector container and the distributor. The overlap reduces the width of the multiple channel flat pipe in the region of connection, that width for circular cross-sections determining the minimum inner diameter of the collector container or distributor.

Because each of the zones that result due to dividing the ends of the multiple channel flat pipes, at its end directed towards the connection is aligned parallel to the main plane of extension of the multiple channel flat pipes, connection between the multiple channel flat pipes, on the one hand, and the collector container and the distributor, on the other hand, can be made in a really known way by inserting the ends in corresponding slits in the walls of the collector container and distributor, then making a fluid-tight connection. The slits required for that have a length distinctly shorter compared with the width of the entire multiple channel flat pipe, and are advantageously arranged crosswise to the longitudinal axis of the collector container or the distributor. Due to this fact it is hardly required to make the wall thicker in order to prevent the slits from spreading when high pressures are applied, and joints between the multiple channel flat pipes and the collector container, or the distributor, respectively, are subjected to lower loads. The longitudinal axes of the collector container and the distributor advantageously run vertically to the main plane of extension of the multiple channel flat pipes.

In many cases it is sufficient and particularly advantageous for manufacture, if the multiple channel flat pipes are divided at their ends in two zones that are shaped such that each of the zones at its end directed toward the connection is aligned parallel to the main plane of extension of the multiple channel flat pipes, while the zones at least partly overlap in direction of the longitudinal axes of the collector container and the distributor.

Particularly, if the multiple channel flat pipes are divided at their ends to form two zones of equal width and both zones at the ends oI the multiple channel flat pipes are shaped such that each of the zones at its end directed toward the connection is aligned parallel to the main plane of extension of the multiple channel flat pipes and the ends of these zones are disposed following each other in direction of the longitudinal axes of the collector container and the distributor, the minimum required inner diameter of the collector container and the distributor is reduced compared with the connection using an non-divided multiple channel flat pipe by half.

Therefore, particularly preferred are multiple channel flat pipes where the ends are divided by a cut in half, with divisions of the multiple channel flat pipes other than in half being possible.

A big advantage of the invention is a potential for reducing the required inner diameter of the collector container and the distributor without appreciably twisting the multiple channel flat pipes. This significantly reduces the danger of overload so that even multiple channel flat pipes made of aluminium may be used.

Moreover, in conjunction with the reduced inner diameter of the collector container and the distributor the invention makes possible to keep the wall thicknesses thereof relatively thin even when to be used at high operational pressures. Therefore also the dimensions and weight of the containers can be kept small. Thus material, weight and production costs are saved. Advantageously, a smaller inner volume of collector container and distributor is associated with a smaller quantity of refrigerant in the system. If the zones of the ends of the multiple channel flat pipes, which are inserted in the collector container and the distributor, are distinctly narrower than the inner diameter of collector container and distributor, they need not be inserted very far in the slits provided in the walls of collector container and distributor for connection. This results in a shorter overhang of the ends of the multiple channel flat pipes, thereby avoiding flow problems particularly an unacceptable refrigerant pressure drop in the distributor.

A connection between the end pipes of a heat exchanger element and the fine channels of the multiple channel flat pipes is created that keeps the diameter of the end pipes necessary to realize said connection relatively narrow without the necessity to considerably enlarge the distance between adjacent multiple channel flat pipes.

Particularly the danger of overloading the multiple channel flat pipes can be reduced by utilizing a process for modifying according to the invention the conventional multiple channel flat pipes and producing a heat exchanger element. The process according to the invention avoids damage to individual channels such as cracks, restrictions or leaks. The process includes at least the following steps:

-   -   dividing the ends of multiple channel flat pipes to form several         zones;     -   spreading the end zones in the main plane of extension of the         multiple channel flat pipes;     -   bending the end zones out of the main plane of extension of the         multiple channel flat pipes;     -   pushing the end zones of the multiple channel flat pipes over         each other, until they overlap at least partly at a pre-given         distance parallel to each other;     -   inserting the end zones of the multiple channel flat pipes in         prepared slits of a collector container and a distributor;     -   connecting the end zones of the multiple channel flat pipes to         the collector container and the distributor.

The connections made in this way between the multiple channel flat pipes and the end pipes, that is between the distributor and the collector container of a heat exchanger element demand less space while affecting the flow cross-sectional area less. Therefore end pipes can be used with smaller diameters while offering the same flow cross-sectional area, which particularly at high operational pressures is associated with a clearly reduced minimum wall thickness. The cutting and bending design according to the invention is, therefore, particularly suitable for gas coolers in air conditioning units of motor vehicles.

Advantageously, the ends of the multiple channel flat pipes are divided in such a way that two zones of equal width are created. The potential of the process according to the invention comes into full effect, if both end zones of the multiple channel flat pipes are pushed over each other so far that both end zones run parallel to each other and to the main plane of extension of the multiple channel flat pipes and the end zones lie behind each other in a direction vertical to the main plane of extension of the multiple channel flat pipes.

It has shown to be advantageous that the connection of the end zones of the multiple channel flat pipes to the collector container and the distributor is made by brazing.

If the end zones of the multiple channel flat pipes are shaped with the aid of a bending gauge, relatively small bending radii can be achieved without damaging the material, preferably aluminium, such as by squeezing, kinking, overstretching, or cracks. A proven method is shaping the end zones of the multiple channel flat pipes making use of at least 6 different small radii bent with the aid of a bending gauge.

The invention is now explained by means of an example of embodiment. The accompanying figures show:

FIG. 1 a schematic representation of a distributor prepared for connection to multiple channel flat pipes according to prior art;

FIG. 2 a schematic representation of a multiple channel flat pipe according to prior art pushed in a traditional distributor;

FIG. 3 a schematic representation of a distributor prepared in an alternative manner for connection to multiple channel flat pipes according to prior art;

FIG. 4 the end of a multiple channel flat pipe after each step of the four invention-relevant steps of the process for producing a heat exchanger element according to the invention;

FIG. 5 a perspective view of two multiple channel flat pipes modified according to the invention, in conjunction with the accompanying distributors;

FIG. 6 a perspective total view of a heat exchanger element according to the invention;

FIG. 7 a view of an exemplary multiple channel flat pipe.

FIG. 1 shows a schematic representation of a distributor 1 of a heat exchanger element according to prior art prepared for connection to multiple channel flat pipes. The distributor 1 is provided with slits 2 arranged crosswise to its longitudinal axis, where multiple channel flat pipes can be pushed in before they are securely brazed to the distributor 1. The slits 2 run parallel, spaced apart at regular distances. Noticeable is that the slits 2 extend over almost the total thickness of the distributor 1.

FIG. 2 shows a schematic representation of a multiple channel flat pipe 3 pushed in a conventional distributor 17 according to prior art. The width of the multiple channel flat pipe 3 approximately corresponds to the inner diameter of the tubular distributor 1. Because the multiple channel flat pipe 3 is inserted crosswise to the cylindrical axis of the distributor 1, deep insertion of the multiple channel flat pipe 3 into the distributor 1 is necessary in order to enable a sealing connection. The substantial restriction of the cross-section required may involve flow problems, especially an inacceptable refrigerant pressure drop in the distributor 1.

FIG. 3 shows a schematic representation of a distributor 1 prepared in an alternative manner for connection to multiple channel flat pipes according to prior art. The distributor 1 is provided with slits 2 arranged parallel to its longitudinal axis, whereby multiple channel flat pipes can be pushed in the slits before they are securely brazed to the distributor 1. Such a design of a distributor is used, if multiple channel flat pipes having end zones twisted by 90° are inserted. Between the slits only narrow webs 4 remain that have to be ensure the stability of the distributor 1 even at high operational pressures in order to not excessively load the brazing seams. The lengths of a slit 2 and a web 4 together make the distance between two adjoining multiple channel flat pipes, which therefore must remain relatively large.

FIG. 4 shows the end of a multiple channel flat pipe after each step of the four invention-relevant steps of the process for producing a heat exchanger element according to the invention, in top and front view. Arrows indicate the direction of each shaping step, reference signs relate to identical components in all four single representations. In step 1.) the ends of the multiple channel flat pipe 3 are divided into two zones 3′, 3″ of equal width, by a slit 5 of 15 mm depth in this case. In step 2.) the end zones 3′, 3″ are spread in the main plane of extension of the multiple channel flat pipe 3. The bending radius of spreading depends upon the depth of the slit 5 separating the zones 3′, 3″, the radius being equal to 33 mm in this example of embodiment. In step 3.) the end zones 3′, 3″ are bent out of the main plane of extension of the multiple channel flat pipe 3, until the ends have a pre-given distance, 3.65 mm in this case, vertical to the main plane of extension of the multiple channel flat pipe 3, this distance corresponding to the distance between individual slits 2 for connecting a multiple channel flat pipe 3 in a distributor 1. In step 4.) the end zones 3′, 3″ of the multiple channel flat pipe 3 are pushed over each other, until they at least partly overlap at the pre-given distance parallel to each other. Preferably, pushing over each other is executed until both end zones 3′, 3″ lie behind each other in a direction vertical to the main plane of extension of the multiple channel flat pipes.

The radii bent in the last step 4.) define for a given slit width also the depth of insertion of the end zones 3′, 3″ of the multiple channel flat pipe 3 in the distributor 1. Particularly, the two bent radii R6 limit for a slit width of 6 mm the insertion depth, in this example, to 5 mm. If these radii were bigger, the end zones 3′, 3″ would plunge deeper than 5 mm into the distributor. This again would reduce the inner cross-sectional area of the distributor.

FIG. 5 shows a perspective view of two multiple channel flat pipes 3 modified according to the invention, in conjunction with the accompanying distributors 1 of the heat exchanger elements. In the front arrangement, a multiple channel flat pipe 3 modified according to the invention is aligned to be inserted in prepared slits 2 of the distributor 1. In the rear arrangement, a multiple channel flat pipe 3 modified according to the invention is already in inserted position, where brazing to the distributor 1 can be carried out. The arrow indicates the direction of movement of the multiple channel flat pipe modified according to the invention during assembly of the heat exchanger element. It is clearly seen that the length of the slits 2, compared with FIG. 1, is reduced relative to the outer periphery of the distributor 1. At the same time, the width of the multiple channel flat pipe 3 is no longer the dimension that limits the inner diameter of the distributor 1 downwards so that the inner diameter can be kept smaller compared with the design in FIG. 2.

FIG. 6 shows a perspective total view of a heat exchanger element according to the invention. The heat exchanger element includes a row of multiple channel flat pipes 3 that are mutually spaced apart, the ends of which are modified according to the invention and connected in a fluid-tight manner to a tubular collector container 6 and a tubular distributor 1. The arrow indicates the direction of flow of the refrigerant, CO₂ in the example.

FIG. 7 shows a view of an exemplary multiple channel flat pipe 3 that serves as starting material for producing heat exchangers according to the invention. It is made of aluminium and has twelve channels 7 regularly distanced to each other through which, during operation, CO₂ as refrigerant passes. Due to the even number of channels it is possible to create by dividing the multiple channel flat pipe 3 between the innermost channels two end zones 3′, 3″ according to the invention that are characterized by equal width. 

1. Heat exchanger element having a row of multiple channel flat pipes (3), the ends of the multiple channel flat pipes (3) being connected in a fluid-tight manner to a tubular collector container (6) and a tubular distributor (1) characterized by that the multiple channel flat pipes (3) at the ends thereof are divided into several zones (3′, 3″) shaped such that each of the zones (3′, 3″) at its end directed to the connection is aligned parallel to the main plane of extension of the multiple channel flat pipes (3) and the zones (3′, 3″) at least partly overlap in direction of the longitudinal axes of the collector container (6) and the distributor (1).
 2. Heat exchanger element to claim 1 characterized by that the multiple channel flat pipes (3) at the ends thereof are divided into two zones (3′, 3″) shaped such that each of the zones (3′, 3″) at its end directed to the connection is aligned parallel to the main plane of extension of the multiple channel flat pipes (3) and the zones (3′, 3″) at least partly overlap in direction of the longitudinal axes of the collector container (6) and the distributor (1).
 3. Heat exchanger element to claim 1 characterized by that the longitudinal axes of the collector container (6) and the distributor (1) run vertical to the main plane of extension of the multiple channel flat pipes (3).
 4. Heat exchanger element to claim 1 characterized by that the multiple channel flat pipes (3) at the ends thereof are divided into two zones (3′, 3″) of equal width.
 5. Heat exchanger element to claim 1 characterized by that both zones (3′, 3″) at the ends of the multiple channel flat pipes (3) are shaped such that each of the zones (3′, 3″) at its end directed to the connection is aligned parallel to the main plane of extension of the multiple channel flat pipes (3), the ends of the zones (3′, 3″) lie behind each other in longitudinal direction of the collector container (6) and the distributor (1).
 6. Heat exchanger element to claim 1 characterized by that the multiple channel flat pipes (3) are made of aluminum.
 7. Heat exchanger element to claim 1 characterized by that the multiple channel flat pipes (3) contain at least twelve single channels (7).
 8. Process for producing a heat exchanger element characterized by that at least the following steps are included: dividing ends of multiple channel flat pipes to form several zones; spreading the end zones in a main plane of extension of the multiple channel flat pipes; bending the end zones out of the main plane of extension of the multiple channel flat pipes; pushing the end zones of the multiple channel flat pipes over each other, until they overlap at least partly at a pre-given distance parallel to each other; inserting the end zones of the multiple channel flat pipes in prepared slits of a collector container and a distributor; connecting the end zones of the multiple channel flat pipes to the collector container and the distributor.
 9. Process to claim 8 characterized by that the ends of the multiple channel flat pipes are divided into two zones.
 10. Process to claim 9 characterized by that the ends of the multiple channel flat pipes are divided into two zones of equal width.
 11. Process to claim 10 characterized by that both end zones of the multiple channel flat pipes are pushed over each other so far that both end zones run parallel to each other and to the main plane of extension of the multiple channel flat pipes and the end zones lie behind each other in a direction vertical to the main plane of extension of the multiple channel flat pipes.
 12. Process to claim 8 characterized by that the connection of the end zones of the multiple channel flat pipes to the collector container and the distributor is made by brazing.
 13. Process to claim 8 characterized by that the end zones of the multiple channel flat pipes are shaped with the aid of a bending gauge.
 14. Process to claim 13 characterized by that shaping the end zones of the multiple channel flat pipes with the aid of the bending gauge is performed by bending with at least 6 different radii. 